Doctor regales son, showing generosity (8)
I believe the answer is:
largesse
'showing generosity' is the definition.
'doctor regales son' is the wordplay.
'doctor' is an anagram indicator.
'son' becomes 's' (genealogical abbreviation for son).
'regales'+'s'='regaless'
'regaless' with letters rearranged gives 'LARGESSE'.
(Other definitions for largesse that I've seen before include "Generous offering" , "Generosity in giving gifts" , "Magnanimity, munificence" , "Magnanimous bestowal of gifts" , "Generous alms" .)